Plant life: Saguaro black spot needs disinfecting - East Valley Tribune: Home

Plant life: Saguaro black spot needs disinfecting

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Posted: Saturday, March 3, 2007 4:48 am | Updated: 5:43 pm, Fri Oct 7, 2011.

Q: I have a large black area on my saguaro cactus that is oozing and smells bad. What is it and how can I treat it?

Q: I have a large black area on my saguaro cactus that is oozing and smells bad. What is it and how can I treat it?

A: It is called bacterial necrosis of saguaro. The bacteria could have entered from a wound, and the first symptom is a small, light-colored spot with a watersoaked margin on the surface of the trunk or branches that may easily go unnoticed. The tissue under the infection site becomes brown or almost black.

As disease progresses, the tissue may crack and exude a dark brown liquid. If decay is slow, the tissue may not drip liquid. As infected tissue breaks down the woody skeleton is exposed. Older infection sites that exude a dark liquid, especially those at the base of the plant, are not treatable. Like most infections, if caught early, successful treatment is greater than when the infection is well-advanced.

Take a large spoon or similar instrument and scoop out the rooted flesh for disposal. Leave the bottom of the hole sloping down toward the outside so water can drain out. The texture of the saguaro is similar to a potato, so keep digging, including a margin of green flesh.

Disinfect the scooping instrument with alcohol after each removal or you will contaminate and re-infect surrounding tissue. You may have heard that 10 percent bleach solution disinfects. Lab and field tests have not shown that to be very effective. Some gardeners have used a Bordeaux paste as a bandage on the open wound, and other gardeners have just left the wound open to air-dry. Birds often make their nests in saguaros without bacterial infection. The tissue just scabs over. There’s no guarantee any treatment will be successful.

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